1. Field of the Invention
The present invention relates to a polishing method and apparatus, and more particularly to a polishing method and apparatus for polishing a workpiece such as a semiconductor wafer to a flat mirror finish.
2. Description of the Related Art
Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections, and also narrower spaces between interconnections which connect active areas. One of the processes available for forming such interconnections is photolithography. Although a photolithographic process can form interconnections that are at most 0.5 xcexcm wide, it requires that surfaces on which pattern images are to be focused by a stepper be as flat as possible because a depth of focus of an optical system is relatively small. However, conventional apparatuses for planarizing semiconductor wafers (substrate-like workpieces), such as self-planarizing CVD apparatus or etching apparatus, fail to produce completely planarized surfaces on semiconductor wafers. Recently, it has been attempted to planarize semiconductor wafers with a polishing apparatus which is expected to easily achieve complete planarization of the semiconductor wafers as compared with the above conventional apparatuses. Such a process is called Chemical Mechanical Polishing (CMP) in which semiconductor wafers are chemically and mechanically polished while supplying a polishing liquid comprising abrasive particles and a chemical solution such as alkaline solution.
FIG. 1 is a schematic view showing a basic structure of this type of polishing apparatus. As shown in FIG. 1, the polishing apparatus has a turntable 1 with a polishing cloth 11 attached thereon and constituting a polishing surface, and a top ring 2 for holding a semiconductor wafer (workpiece) W in such a manner that a surface, to be polished, of the semiconductor wafer W faces the turntable 1. The top ring 2 is connected to a lower end of a top ring shaft 3 via a ball joint 4 so as to be tiltable with respect to the top ring shaft 3. The semiconductor wafer W to be polished is pressed against the turntable 1 under a certain pressure by the top ring 2 while the turntable 1 and the top ring 2 are independently being rotated, and the surface of the semiconductor wafer W is polished to a flat mirror finish while a polishing liquid Q is being supplied from a polishing liquid supply nozzle 5. In this case, the surface, to be polished, of the semiconductor wafer W is brought into sliding contact with an upper surface of the polishing cloth 11 while following an inclination of the upper surface of the polishing cloth 11 via the ball joint 4.
As a polishing cloth attached on a turntable, a polishing cloth made of non-woven fabric has heretofore been employed. Higher levels of integration achieved in recent years for ICs and LSI circuits demand smaller steps or surface irregularities on a polished surface of a semiconductor wafer. In order to meet such a demand, there has been used a polishing cloth made of a hard material such as polyurethane foam.
After a semiconductor wafer W is polished by the polishing apparatus, it is necessary to remove the semiconductor wafer W from the polishing surface (the polishing cloth 11) on the turntable 1. However, a large surface tension acts between the polishing cloth 11 and the semiconductor wafer W due to polishing liquid Q interposed therebetween. Accordingly, if the top ring 2 holding the semiconductor wafer W is lifted at a polishing position in order to remove the semiconductor wafer W from the polishing cloth 11, there are some cases in which only the top ring 2 is lifted and the semiconductor wafer W adheres to the polishing cloth 11 so at to be left on the polishing cloth 11.
Such a problem can be solved by an overhanging action of the top ring. In the overhanging action, after the polishing process is completed, the top ring 2 is not lifted at the polishing position, but is moved to an outer circumferential edge of the polishing cloth 11 to partly expose a polished surface of a semiconductor wafer W beyond the outer circumferential edge of the polishing cloth 11, and is then lifted to remove the semiconductor wafer W from the polishing cloth 11. This overhanging action allows surface tension between the polishing cloth 11 and the-semiconductor wafer W to be reduced, and the semiconductor wafer W can reliably be separated or removed from the polishing cloth 11.
As described above, with the overhanging action, the surface tension between the polishing cloth 11 and the semiconductor wafer W can be reduced. However, the top ring 2 may tilt when the polished semiconductor wafer W projects from the outer circumferential edge of the polishing cloth 11. In this case, the semiconductor wafer W is intensively pressed at the outer circumferential edge of the polishing cloth 11, so that the semiconductor wafer W is cracked or scratched.
Polishing capability of a polishing cloth is gradually deteriorated due to a deposit of abrasive particles and ground-off particles of semiconductor material, and due to changes in the characteristics of the polishing cloth. Therefore, if the same polishing cloth is used to repeatedly polish semiconductor wafers, a polishing rate of the polishing apparatus is lowered, and polished semiconductor wafers tend to suffer polishing irregularities. Therefore, it has been customary to condition the polishing cloth according to a process called xe2x80x9cdressingxe2x80x9d for recovering a surface of the polishing cloth with a diamond dresser or the like before, after, or during polishing.
When a diamond dresser dresses a polishing surface of polishing cloth 11, it scrapes a thin layer off the polishing cloth 11. Therefore, after the polishing surface of the polishing cloth has been dressed many times, it becomes irregular, i.e. loses its planarity, thereby causing formation of steps. As a result, during movement of a polished semiconductor wafer W to the outer circumferential edge of the polishing cloth 11 in the aforementioned overhanging action, the semiconductor wafer W may be cracked or scratched because of the irregularities of the polishing cloth 11.
The present invention has been made in view of the-above drawbacks. It is therefore an object of the present invention to provide a polishing method and apparatus which can easily and safely remove a polished workpiece from a polishing surface, and can increase throughput.
According to an aspect of the present invention, there is provided a polishing method comprising: rotating a turntable having a polishing surface thereon, and a top ring for holding a workpiece to be polished, respectively; pressing a surface, to be polished, of the workpiece held by the top ring against the polishing surface on the turntable to polish the workpiece; and increasing a relative speed of the turntable and the top ring when a polished workpiece held by the top ring is to be removed from the polishing surface on the turntable, as compared to a relative speed of the turntable and the top ring at a period of polishing.
According to a preferred aspect of the present invention, a rotational speed of the turntable is increased as compared to a rotational speed of the turntable at a period of polishing to increase the relative speed of the turntable and the top ring.
According to another preferred aspect of the present invention, rotational speeds of the turntable and the top ring are respectively increased as compared to rotational speeds of the turntable and the top ring at a period of polishing to increase the relative speed of the turntable and the top ring.
According to another aspect of the present invention, there is provided a polishing apparatus for polishing a workpiece to be polished, comprising: a turntable having a polishing surface thereon; a top ring for holding the workpiece to be polished and pressing the workpiece against the polishing surface on the turntable; a motor for rotating the turntable; and a controller for controlling a rotational speed of the motor, wherein when a polished workpiece held by the top ring is to be removed from the polishing surface on the turntable, the motor increases a rotational speed of the turntable as compared to a rotational speed of the turntable at a period of polishing.
According to still another aspect of the present invention, there is provided a polishing apparatus for polishing a workpiece to be polished, comprising: a turntable having a polishing surface thereon; a top ring for holding the workpiece to be polished and pressing the workpiece against the polishing surface on the turntable; a motor for rotating the top ring; and a controller for controlling a rotational speed of the motor, wherein when a polished workpiece held by the top ring is to be removed from the polishing surface on the turntable, the motor increases a rotational speed of the top ring as compared to a rotational speed of the top ring at a period of polishing.
According to another aspect of the present invention, there is provided a polishing method comprising: moving a polishing surface and a top ring holding a workpiece to be polished relative to each other; pressing a surface, to be polished, of the workpiece held by the top ring against the polishing surface to polish the workpiece; and increasing a relative speed of the polishing surface and the top ring when a polished workpiece held by the top ring is to be removed from the polishing surface, as compared to a relative speed of the polishing surface and the top ring at a period of polishing.
It is desirable that the rotational speed of the turntable be increased to at least 100 revolutions per minute, and the rotational speed of the top ring be increased to at least 75 revolutions per minute.
According to the present invention, a liquid film thickness of a polishing liquid on the polishing surface is decreased by action of centrifugal force, so that surface tension due to the polishing liquid is reduced. Therefore, even if the top ring is lifted at the polishing position without an overhanging action, a workpiece can easily be removed from the polishing surface. Thus, the present invention can prevent the workpiece from being left on the polishing surface and being cracked or scratched. Further, since it is not necessary to perform the overhanging action, tact time for polishing can be reduced to increase throughput. Furthermore, when the rotational speed of the turntable is increased, components of the polishing liquid are scattered and discharged from an upper surface of the turntable by action of centrifugal force. Accordingly, it becomes easier to remove the workpiece from the polishing surface, and simultaneously polishing conditions for a subsequent workpiece to be polished can be arranged to be preferred conditions. As a result, throughput can be increased.
The above and other objects, features, and advantages of the present invention will be apparent from the following description when taken in conjunction with the accompanying drawings which illustrates preferred embodiments of the present invention by way of example.